Research On Methods For Residual Deformation Minimization And Topology Design With Feature Size Control For Additive Manufacturing Structures

Additive manufacturing(AM) which is a revolutionary manufacturing technology is developing rapidly in recent years, its development and application has gained great attention in China and abroad. Additive manufacturing fabricates parts using an additive approach as opposed to traditional manufacturing technology, thus it can achieve free forming of complex geometric shapes, which broadens the design space of structure and makes performance oriented rather than manufacturability oriented design possible. Topology optimization is an effective tool for innovative design of advanced structures, it can achieve automatic design of the optimum structure in the design space according to the performance requirements. So combining AM(an advanced manufacturing technology) and topology optimization(an advanced structural design method) is of great significance to the development of advanced materials and structures. It is needed to study the problems in the combination of them. For example, feature size control should be introduced into the structure design method considering the manufacturability requirement given by AM. The manufacturing process should be designed in order to alleviate the residual deformation of the structure. This paper studies the influence of the scanning pattern on the residual deformation of additive manufacturing structures, proposes methods for designing the scanning pattern and support position with minimized residual deformation; proposes methods for imposing minimum and maximum length scale in topology optimization considering the influence of the manufacturability requirement on the optimal design. So this paper provides a certain basis for the combination of AM and topology optimization. The main contents and results are as follows.(1) Scanning pattern design with minimized residual deformation. In many additive manufacturing processes, the moving local heat source inevitably leads to inhomogeneous temperature distribution, which causes residual deformation of the parts due to high temperature gradient. In order to alleviate residual deformation, a numerical model which simulates the process of scanning the top surface of the central region of a flat plate using a moving laser source is used, by which the effect of laser scanning pattern on the residual deformation is investigated. A two-step method of designing scanning pattern with minimized residual deformation based on dividing the scanning region into sectors is proposed. In this method, the number, size and scanning sequence of the sectors are firstly designed, then the scanning pattern in each sector is designed. The optimized scanning pattern for certain design examples is obtained by using the proposed method. The results show that the residual deformation is greatly alleviated by designing a optimized scanning pattern.(2) Support position design with minimized residual deformation. Support structure is needed in many additive manufacturing processes, one of the reasons is that support structure can alleviate residual deformation of the parts. In order to ease support removal, point connections between the main support and the part are usually used. In order to alleviate residual deformation, The optimization problem of support position of the point connections is posed. To deal with the problem that large number of nonlinear thermal mechanical coupling analyses are needed in the optimization process, a method of building equivalent simplified model of the real model is proposed. By using the equivalent model in the optimization process, the computational cost can be decreased substantially. Considering the mismatch of the equivalent model and the real model, an iterative optimization design method which corrects the equivalent model iteratively is proposed. The effectiveness of the method is showed through certain design examples.(3) Projection based method for imposing maximum and minimum length scale in topology optimization. After decades of development, topology optimization has been successfully applied to the design of the optimal engineering structures. However, the optimized topologies are often considered as conceptual due to ignoring of manufacturing constraints in the topology optimization model. As one of important manufacturing constraints, a large effort has been devoted to the issue of length scale control which consists of minimum length scale control and maximum length scale control in topology optimization. In this paper, a novel projection based method for imposing maximum length scale in topology optimization is presented. In this method, a new penal formulation for the density of the center element which violates the maximum length scale constraints is proposed and the maximum length scale control can be achieved without additional constraints. Furthermore, a combination of the new method and the robust formulation which is able to control the minimum length scale is realized by changing the penal formulation into a global constraint. By this way, simultaneously control of the minimum length scale and maximum length scale is achieved. Additionally, projection based methods for considering symmetry constraint, pattern repetition constraint and maximum overhang constraint in topology optimization have been simply studied. The validity of the methods is validated through some design problems.(4) Study on the optimization process of topology optimization aiming at improving optimized designs. At present, topology optimization can not ensure global optimum solution. Unsatisfactory results may occur in some problems. To deal with the problem, the optimization process of topology optimization is studied through analysis of a design problem with multiple initial solutions. Three ways of evolution of design in the optimization process are pointed out, among which evolution of ground structure with intermediate density will not exist when the density field approaches to discrete 0-1 values. In order to take advantage of the ground structure again, an optimizing strategy of reusing ground structure is proposed and the strategy is realized by using a proposed method for controlling the evolution of design. Numerical examples show that the strategy can improve unsatisfactory optimized results in topology optimization.